An economical and efficient method for separating triglycerides on the basis of degree of unsaturation has previously been sought to satisfy commercial pressures. For example, in Ou U.S. Pat. No. 4,961,881, the desirability of reducing the level of unsaturated fatty acid groups in synthetically produced triglycerides was disclosed since the product could be used as a cocoa butter extender. In view of more recent trends to reduce the monounsaturated components of triglyceride mixtures for health reasons, the applications of such a process in edible products, such as margarine, mayonnaise, etc., are apparent.
Thus, the value of available feed materials such as soybean oil, cottonseed oil, linseed oil, corn oil, peanut oil, sunflower oil, safflower oil, canola oil, olive oil, rich bran oil, sesame, and almond, etc., can be enhanced by processing to give fractions which are enriched or deleted in unsaturation. Other highly saturated feeds, such as tallow, lard, coconut, palm oil, etc., may be reacted with unsaturated fatty acids via an interesterification process, as disclosed in U.S. Pat. Nos. 4,275,081 or 3,328,439, to increase the degree of unsaturation and the product thereof can be separated by the process of the invention.
The separation of many classes of compounds by selective adsorption on molecular sieves or zeolites as well as other adsorbents is well known. Also, various separations based on the degree of unsaturation are known, e.g., esters of saturated fatty acids from unsaturated fatty acids with X or Y zeolites exchanged with a selected cation, from U.S. Pat. No. 4,048,205, monoethanoid fatty acids from diethanoid fatty acids with cross-linked polystyrenes, e.g., "Amberlite", from U.S. Pat. No. 4,353,838. A process for separating a mixture of triglycerides, based on the iodine values, is shown in U.S. Pat. Nos. 4,277,412 and 4,284,580 in which permutite and surface-aluminated silica gel adsorbents, respectively, can be used. However, both of these require silver-exchanged surface-aluminated silica gel adsorbents, which is not only undesirable in food product preparation, but rapid fouling of these adsorbents by any impurities in the feed mixtures has limited commercial application of these materials. The adsorbents of the invention are not subject to the limitation of these prior art materials. Ou U.S. Pat. No. 4,961,881 describes a process for overcoming the deactivation of the surface-treated silica gel of 4,284,580 by continuously or intermittently regenerating the adsorbent with hydrogen peroxide or an organic peroxide. The improved adsorbents of the present invention are remarkably stable and thus do not exhibit the rapid deactivation that is exhibited by the prior art absorbents.
The adsorption properties of silane- and/or alcohol-treated silica gels have been reported and found useful in certain analytical separations, e.g., thin layer chromatography (TLC). For example, Plattner et al, Lipids 14 (2), (1979), pp 152-3 reported that triglycerols could be separated by both chain length and number of double bonds with reverse phase columns, i.e., .mu.-Bondapak C.sub.18 or .mu.-Porasil silica gels with octadecyl silyl groups bonded to silica particles. Also, Plattner et al, JAOCS 54 (11) (November 1977) pp 511-15. Acetonitrile: acetone (2:1 v/v) mixtures were used as elution agents. Neither, however, describes a process capable of separating triglycerides by degree of unsaturation in bulk quantities nor the preferred desorbents of the present invention.
A process for making surface-silanized TLC separation materials for use in aqueous elution agent systems is disclosed in U.S. Pat. Nos. 4,741,830 and 4,793,921, incorporated by reference herein.
Japanese Public Disclosure No. 192797/86 discloses a method for concentrating eicosapentaenoic acid and docosahexaenoic acid in their triglyceride forms with silica gel chemically bound with an octadecyl group or a styrene-divinylbenzene copolymer.
Zinnen U.S. Pat. No. 4,784,807 discloses the separation of triglycerides based on degree of unsaturation with omega zeolite or carbon adsorbents and ketones, toluene and ketone/aliphatic hydrocarbon mixtures as desorbents.
The invention herein can be practiced in fixed or moving adsorbent bed systems, but the preferred system for this separation is a countercurrent simulated moving bed system, such as described in Broughton U.S. Pat. No. 2,985,589, incorporated herein by reference. Cyclic advancement of the input and output streams can be accomplished by a manifolding system, which are also known, e.g., by rotary disc valves showing in U.S. Pat. Nos. 3,040,777 and 3,422,848. Equipment utilizing these principles are familiar, in sizes ranging from pilot plant scale (deRosset U.S. Pat. No. 3,706,812) to commercial scale in flow rates from a few cc per hour to many thousands of gallons per hour.
The functions and properties of adsorbents and desorbents in the chromatographic separation of liquid components are well known, but for reference thereto, Zinnen et al U.S. Pat. No. 4,770,819, which relates to the separation of diglycerides from triglycerides with omega zeolite or silica adsorbents is incorporated herein. From FIG. 1 and Example II of this patent, it can be seen that Zinnen et al was not able to separate triglycerides on the basis of degree of unsaturation with silica gel, since all the triglycerides eluted at the same time.
I have found adsorbents, which, in combination with certain desorbent liquids, will selectively adsorb the more highly unsaturated triglycerides contained in triglyceride mixtures having components with different degrees of unsaturation. The more saturated (less unsaturated) triglycerides are relatively non-adsorbed and elute first. Thus, the more saturated triglyceride components of the feed are eluted as raffinate and the more highly unsaturated triglycerides are adsorbed and eluted as extract by desorption with the desorbent.
These adsorbents are thermally stable and thus can be regenerated easily at elevated temperatures without collapsing the pore structure. Furthermore, since there are no metal exchange ions, they are deemed suitable for the separation of food products and are chemically stable to impurities contained in the feed.
I have discovered a method for separating mixtures of unsaturated triglycerides on the basis of degree of unsaturation. The triglycerides may include monounsaturated and polyunsaturated triglycerides.